Since the method for mass production of fullerene C60 has been established in 1990, intensive studies on fullerene have been continuously performed. As a result, a large number of fullerene derivatives have been synthesized, and various functions and properties thereof have been discovered. In consequence, various uses and applications of the fullerene derivatives have been developed (refer to “Gendai Kagaku”, April, 1992, p. 12, “Gendai Kagaku”, June, 2000, p. 46, and “Chemical Reviews”, 98, p. 2527 (1998)).
The present inventors have already synthesized and reported various penta-addition fullerene derivatives (Japanese Patent Application Laid-open (KOKAI) Nos. 10-167994, 11-255509 and 2002-241323, “J. Am. Chem. Soc.”, 118, p. 12850 (1996), “Org, Lett.”, 2, p. 1919 (2000), and “Chem. Lett.”, p. 1098 (2000)). These fullerene derivatives of C60 skeleton have a π-electron conjugated structure of a 50 electron system, and are different in steric configuration and electronic properties from C60 itself having a π-electron conjugated structure of a 60 electron system. Therefore, it has been expected that the above penta-addition fullerene derivatives provide new electronic conductive materials, semiconductors and bioactive substances, etc.
In addition, there are known deca-addition fullerene derivatives that are larger in number of addition of organic groups than that of the penta-addition fullerene derivatives (refer to Japanese Patent Application Laid-open (KOKAI) No. 2002-241323). However, the deca-addition fullerene derivatives of C60 skeleton have a π-electron conjugated structure of a 40 electron system, and, therefore, are largely different in electron state thereof from those of the fullerene itself and penta-addition fullerene derivatives. Further, the present inventors have successfully synthesized and reported triaddition C70 derivatives having a π-electron conjugated structure of a 66 electron system which are smaller in number of addition of organic groups than that of the penta-addition fullerene derivatives (refer to Japanese Patent Application Laid-open (KOKAI) No. 11-255508).
Various fullerene derivatives mentioned above have such a unique structure that organic groups are exclusively added to specific sites of fullerene skeleton, and have the long π-electron conjugated structure. For this reason, there have been raised much interests in electrochemical properties of these fullerene derivatives.
In order to apply the above fullerene derivatives to electronic materials, ligands of metal complexes, etc., and use these derivatives as intermediate products for other fullerene derivatives, the fullerene derivatives preferably show a high solubility in organic solvents. Although some of the penta-addition fullerene derivatives have a higher solubility in organic solvents than that of the unsubstituted fullerene, it has been further required to provide the fullerene derivatives capable of exhibiting a still higher solubility in various organic solvents.
Also, it has been reported that, for example, C60(Ph)5H suffers from oxidation during storage (“Chem. Commun.”, p. 1579 (1997)). Thus, in general, these multiple-addition fullerene derivatives tend to be unstable in air.